Stuff like legal papers and other unique items should go in your safe deposit box. Stuff like CD-ROMS should go there too, but make multiple copies so you can put them in geographically dispersed locations (e.g ask a friend in some distant city to keep a disc or two for you in his or her own SD box, and you can do the same for him/her). Use GnuPG (www.gnupg.org) to encrypt the contents if you are concerned about its privacy. I'd skip the underground buried treasure approach unless you're planning to store something much different than papers or CD's (e.g. survival cache), and if that's the case, I'd avoid putting anything in it that would cause you a lot of trouble if someone else found it.

The best currently available brand of CD for long term storage is Mam-A (Mitsui) Archive Gold which is about $1.60 a disc (Google search). Other types of CD-R cost a lot less but the dyes are less stable. Use 74 minute (650 MB) discs in preference to 80 minute ones, which cram the tracks closer together, decreasing reliability, and record at 8x speed or slower. However, no matter what kind of discs you use, your best safeguard is multiple redundant copies.

See http://www.taobackup.com for more than you wanted to know about backup strategy.